1. Field of the Invention
The present invention relates to an optical multiplexer/demultiplexer.
2. Description of the Related Art
Various kinds of structures of an optical multiplexer/demultiplexer have been proposed in the field of optical communications. FIG. 1 shows the structure of a conventional optical multiplexer/demultiplexer using a dielectric multilayer filter, in which a clad is cross-sectioned along a plane passing an upper face of a core. According to the optical multiplexer/demultiplexer 1, an optical waveguide 4 is constituted such that cores 3a, 3b and 3c are buried in a clad 2 made of a transparent material. In the optical waveguide 4, a groove 6 for a filter 5 such as a dielectric multilayer filter is formed, and the filter 5 is inserted in the groove 6. The first and second cores 3a and 3b are provided in the clad 2 positioned at one side of the groove 6, and are connected in the approximate V-shape at the end facing the groove 6. In addition, the third core 3c is provided in the clad 2 positioned at the other side of the groove 6 such that its end is opposed to the end of the first core 3a. 
In addition, an optical fiber 7a is coupled to an end face of the first core 3a; an optical fiber 7b is coupled to an end face of the second core 3b; and an optical fiber 7c is coupled to an end face of the third core 3c. 
It is assumed herein that the filter 5 used in this optical multiplexer/demultiplexer 1 has characteristics of making the light of wavelengths λ1 and λ2 transmit therethrough and making the light of a wavelength λ3 reflect therefrom among the light of the wavelengths λ1, λ2 and λ3 (λ1<λ2<λ3), for example. According to the optical multiplexer/demultiplexer 1, as shown by arrows of solid line in FIG. 1, when the light of the wavelength λ2 and the light of the wavelength λ3 are simultaneously made incident from the optical fiber 7a onto the first core 3a, for example, the light propagated in the first core 3a and emitted from the end face thereof opposed to the filter 5 is divided into the second core 3b and the third core 3c by the filter 5. That is, among the light emitted from the first core 3a, the light of the wavelength λ2 transmits through the filter 5, in made incident onto the third core 3c, is propagated the third core 3c and is coupled to the optical fiber 7c. At the same time, the light of the wavelength λ3 emitted from the end face of the first core 3a is reflected from the filter 5, is made incident onto the second core 3b, is propagated in the second core 3b is and coupled to the optical fiber 7b. 
In addition, as shown by arrows of broken line in FIG. 1, when the light of the wavelength λ1 is made incident from the optical fiber 7c onto the third core 3c, the light of the wavelength λ1 is propagated in the third core 3c, is emitted from the end face thereof opposed to the filter 5, transmits through the filter 5, is made incident onto the first core 3a, is propagated in the first core 3a and is coupled to the optical fiber 7a. 
According to the above-described optical multiplexer/demultiplexer 1, as shown FIG. 2 in which an X part in FIG. 1 is enlarged, the end of the first core 3a and the end of the second core 3b are connected in approximate V-shape, and both cores 3a and 3b are overlapped with each other over a distance L shown in FIG. 2. The light of the wavelengths λ1, λ2 and λ3 is all single-mode light, and the cores 3a , 3b and 3c propagate the light of the wavelengths λ1, λ2 and λ3 in the single mode. Therefore, when the light of the wavelength λ2 and the light of the wavelength λ3 are made incident from the optical fiber 7a onto the first core 3a, the light is emitted from the first core 3a and, then, a part of the light of the wavelength λ2 which is to transmit through the filter 5 and to be made incident onto the third core 3c enters the core 3b side in a region where the cores 3a and 3b are overlapped, so that the light is disadvantageously made incident onto the second core 3b in some cases. When this phenomenon occurs, the light of the wavelength λ2 is mixed in the second core 3b which is provided for extracting only the light of the wavelength λ3, so that the light of the wavelength λ2 is coupled to the optical fiber 7b and becomes noise, which causes an adverse effect in which communication is hindered, for example.
Similarly, when the light of the wavelength λ1 is made incident from the optical fiber 7c onto the third core 3c, a part of the light emitted from the end face of the third core 3c and transmitting through the filter 5 is not led to the first core 3a but made incident onto the second core 3b, so that it is propagated in the second core 3b and becomes noise, which lowers communication quality in some cases.
In order to reduce the above-described noise, there is known that a branching angle θ between the first and second cores 3a and 3b is increased and isolation between both cores 3a and 3b is enhanced by shortening the overlapped portion of the cores 3a and 3b (the portion shown by the distance L in FIG. 2). FIG. 3 shows a result obtained by making the light of the wavelength λ3 of power P1 incident from the first core 3a onto the filter 5, measuring a power P3 of the light made incident onto the third core 3c and measuring its loss:−10 log [(P1−P3)/P1] (dB).In FIG. 3, a vertical axis designates loss and a lateral axis designates a position (displacement) ξ of the filter 5, in which a filter position showing a minimum loss is a reference point. In addition, in FIG. 3, a thick solid line shows the loss when the branching angle θ is 16° and a thin solid line shows the loss when the branching angle θ is 24°. As can be clear from the result shown in FIG. 3, the minimum value of loss (loss when ξ=0) can be made smaller if the branching angle θ is made larger, so that noise is not likely to leak to the core 3b. 
However, since a curve showing variation in loss becomes abrupt when the branching angle θ becomes large. According to the method of increasing the branching angle θ in order to reduce the noise, therefore, loss is largely varied by a slight variation of the filter position in the groove 6, so that the required position precision of the filter 5 becomes strict and it becomes difficult to manufacture the optical multiplexer/demultiplexer.
In addition, since it is necessary to align the end of the first core 3a and the end of the second core 3b in parallel so that optical axes may coincide with that of the optical fibers 7a and 7b, on the coupling side to the optical fibers 7a and 7b, when the branching angle θ between the cores 3a and 3b is increased, it is necessary to largely curve the first and second cores 3a and 3b in the middle for that. However, since a maximum curvature in which the cores 3a and 3b can be bent while keeping the light confined has been decided, the cores 3a and 3b cannot be bent below the predetermined curvature radius. Therefore, as the branching angle θ is increased, a length of the curved portion of each of the cores 3a and 3b has to be increased, so that the optical waveguide becomes large, cost of the optical waveguide is increased and propagation loss is also increased.